Add quantization algorithms to reserved attributes

[sfinkens]

Fix cfchecker reporting invalid quantization attributes by adding three new reserved attributes.

Starting with version 4.9.0, libnetcdf offers three quantization algorithms. When enabled, they add one of the following attributes to the quantized variable

Algorithm	Attribute
Bit Groom	_QuantizeBitGroomNumberOfSignificantDigits
Granular Bit Round	_QuantizeGranularBitRoundNumberOfSignificantDigits
Bit Round	_QuantizeBitRoundNumberOfSignificantBits

I also added a small test file for this:

netcdf quantization {
dimensions:
        x = 3 ;
variables:
        double bit_groom(x) ;
                bit_groom:_QuantizeBitGroomNumberOfSignificantDigits = 2 ;
                bit_groom:_FillValue = NaN ;
                bit_groom:standard_name = "air_temperature" ;
                bit_groom:units = "K" ;
        double bit_round(x) ;
                bit_round:_QuantizeBitRoundNumberOfSignificantBits = 2 ;
                bit_round:_FillValue = NaN ;
                bit_round:standard_name = "air_temperature" ;
                bit_round:units = "K" ;
        double granular_bit_round(x) ;
                granular_bit_round:_QuantizeGranularBitRoundNumberOfSignificantDigits = 2 ;
                granular_bit_round:_FillValue = NaN ;
                granular_bit_round:standard_name = "air_temperature" ;
                granular_bit_round:units = "K" ;

// global attributes:
                :comment = "Test file with quantized data variables" ;
                :Conventions = "CF-1.8" ;
}